1. Field of the Invention
The present invention relates to a sheet processing apparatus for performing a post-processing on a folded portion of a stacked sheet bundle.
2. Description of the Related Art
Conventionally, there exist image forming apparatuses such as copying machines, printers, and multifunction peripherals which are equipped with a sheet processing apparatus for performing folding processing on sheets on which images have been formed by an image forming unit. However, a sheet bundle that has undergone folding processing can swell. Accordingly, in a conventional sheet processing apparatus, in order to improve the quality of the folded portion and stacking property, flattening processing is performed after the folding of a sheet bundle to flatten a curved top of the folded portion, which is formed through the folding of the sheet bundle.
To flatten the fold top portion, the sheet processing apparatus conveys the folded sheet bundle by a conveyance roller, and causes the fold top portion to strike a positioning member to set it in position. Next, the sheet processing apparatus holds a vicinity of the fold top portion of the sheet bundle by a grasping member to fix it in position, and then retreats the positioning member; after this, a press contact roller travels along the fold top portion while in press contact with the fold top portion of the sheet bundle, thereby flattening the fold top portion.
In such a sheet processing apparatus, when there is variation in the position where the fold top portion of the sheet bundle stops, an amount by which the press contact roller enters the sheet bundle varies, resulting in variation of the configuration of the fold top portion. Therefore, in the sheet processing apparatus as discussed in Japanese Patent Application Laid-Open No. 2006-036493, the sheet bundle is caused to strike a positioning member, thereby enhancing the precision of the stopping position of the sheet bundle.
However, in the case where the sheet bundle is stopped by providing a positioning member, it is necessary to provide the positioning member, a motor for causing the positioning member to retreat, etc. This results in an increase in the cost of the sheet processing apparatus and complicates its structure. On the other hand, in the case where the sheet bundle is stopped without providing any positioning member, depending upon the length of the sheet bundle, the trailing edge of the sheet bundle may stop near the nip portion of conveyance rollers. In this case, the sheet bundle is pushed out to the downstream side by the pressure of the nip portion of the conveyance rollers, so that the stopping position of the sheet bundle may be deviated. In the following, the deviation of the stopping position of the sheet bundle due to the pushing-out will be discussed.
The larger the number of sheets constituting the sheet bundle, the larger the gap of the conveyance roller pair due to the increase in the thickness of the sheet bundle. On the other hand, the trailing edge of the sheet bundle folded in the middle is of a triangular configuration protruding toward the upstream side. If the trailing edge of the sheet bundle is near the nip of the conveyance rollers when the sheet bundle is stopped, the gap of the conveyance roller pair is eliminated, so that the conveyance roller pair pushes out the triangular portion at the trailing edge of the sheet bundle. As a result, the positional relationship between the press contact roller and the sheet bundle is deviated, resulting in variation of the configuration of the fold top portion.
To solve the above problem, if the distance between the stopping position of the sheet bundle and the conveyance rollers on the downstream side is not less than half the maximum sheet length which enables double folding, it might be possible to prevent the pushing-out phenomenon of the conveyance roller pair at the time of stopping the sheet bundle. However, that would involve an increase in the length of the conveyance path, resulting in an increase in the apparatus size.
It is also possible to separate the conveyance rollers near the trailing edge of the sheet bundle. In this case, it is necessary to separate the conveyance rollers after the sheet bundle has reached conveyance rollers on the downstream side of conveyance rollers near the trailing edge of the sheet bundle. To determine whether the sheet bundle has reached the downstream conveyance rollers, an optical sensor can be added to detect the position of the sheet bundle. However, that would require a sensor, a structure for mounting the sensor, etc., resulting in an increase in cost and complicating a structure.
Further, instead of the above method, it is possible to separate the conveyance rollers when a fixed period of time has elapsed after the start of the folding of the sheet bundle in the middle. However, the load on the motor at the time of folding the sheet bundle in the middle varies depending upon the number of sheets constituting the sheet bundle, a type of paper used, etc., resulting in fluctuation in the required time for folding the sheet bundle in the middle. As a result, if, for example, the load is large, and the required time for folding in the middle is long, the upstream side conveyance rollers are separated before the sheet bundle reaches the downstream conveyance rollers, so that the sheet bundle may not be normally conveyed.